so that runtime/race tests are included in the race builder.

Update #14011.

Change-Id: I04ac6e47366fdb1fe84ba89da556c6d38f7d4a47
Reviewed-on: https://go-review.googlesource.com/18686
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>